PT145 · S2 · Q10 One year ago, a municipality banned

We don't care about people who are still using phosphate detergents. We're trying to ascertain whether any residents stopped using phosphate detergents. So interviewing the people who still use detergents won't help us at all.

We're trying to ascertain what caused the treatment plant's phosphate pollution to go down. Phosphate-free detergents don't contribute to "phosphate pollution", so who cares what other sort of pollutants they may have? We are investigating whether anyone switched to phosphate-free detergents. Telling us the ingredients of those detergents doesn't help us figure out whether anyone switched to using them.

We're trying to figure out why less phosphate pollution came out of the plant last year. This answer is about where the water goes after it's come out of the plant, and whether it ends up being a significant source of something somewhere else. All that matters to this question is solving the causal mystery of why less phosphate pollution came out of the treatment plant. Is it because less phosphate entered the treatment plant? If so, why did less enter? Or is it because the treatment plant removed more phosphate while the wastewater was inside the plant? Once this water exits the plant and we measure the plant's output of phosphate as lower than before, we're done with caring about it.

If officials did try to stop people from bringing detergents w/ phosphates into the municipality, then that would mildly strengthen the plausibility that some residents probably switched to phosphate-free detergents. But if we say, "No, officials didn't try to stop people from bringing illegal detergents into the city", it doesn't weaken. It's perfectly plausible that some residents might switch to phosphate-free detergents, given that one year ago the phosphate-variety was banned, with or without official trying to stop rulebreakers from crossing the border with their contraband detergent. The objection we get from (C) is that, "the treatment plant is emitting less phosphate because they improved their filtering processes this year, not because residents switched to phosphate-free." The objection we would get from (E) is that, "it's not plausible that some residents switched to phosphate-free detergents, because the city officials didn't try to stop people from bringing phosphate detergents into the city."